Semiconductor device and driving method thereof

ABSTRACT

A semiconductor device including photosensor capable of imaging with high resolution is disclosed. The semiconductor device includes the photosensor having a photodiode, a first transistor, and a second transistor. The photodiode generates an electric signal in accordance with the intensity of light. The first transistor stores charge in a gate thereof and converts the stored charge into an output signal. The second transistor transfers the electric signal generated by the photodiode to the gate of the first transistor and holds the charge stored in the gate of the first transistor. The first transistor has a back gate and the threshold voltage thereof is changed by changing the potential of the back gate.

TECHNICAL FIELD

The present invention relates to a semiconductor device including aphotosensor and a driving method thereof. The present invention alsorelates to a semiconductor device in which photosensors are arranged ina matrix and a driving method thereof. Further, the present inventionrelates to a display device including a photosensor and a driving methodthereof. The present invention also relates to a display device in whichpixels each including a photosensor are arranged in a matrix and adriving method thereof. In addition, the present invention relates to anelectronic appliance including the display device or the semiconductordevice.

BACKGROUND ART

In recent years, display devices on which a sensor that detects light(also referred to as a “photosensor”) is mounted have attractedattention. By providing a display device with a photosensor, input ofinformation can be performed on a display screen. For example, a displaydevice having an image capturing function can be given (for example, seePatent Document 1).

In addition to the above display device, as a semiconductor deviceincluding a photosensor, an imaging device which is used in anelectronic appliance such as a scanner or a digital still camera can begiven.

In a semiconductor device including a photosensor such as the abovedisplay device or the imaging device, the photosensor detects lightreflected by an object or light emitted from an object; thus, thesemiconductor device can detect the presence of the object around aregion in which the photosensor is provided.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2001-292276

DISCLOSURE OF INVENTION

In order that an object to be detected be imaged and an image beobtained, light needs to be converted into an electric signal in aphotosensor. Since the electric signal is an analog signal in general,the electric signal needs to be converted into a digital signal by anA/D converter circuit. Further, A/D conversion in accordance with theintensity of light needs to be performed.

Therefore, an object of an embodiment of the present invention is toaccurately convert light into an electric signal in a photosensor.Another object is to provide a photosensor with a novel circuitstructure for achieving the above object. Another object is to provide asemiconductor device including the photosensor.

Further, another object is to provide a photosensor capable of imagingwith high resolution. Another object is to provide a semiconductordevice including the photosensor.

Another object is to provide a semiconductor device including aphotosensor capable of imaging an object to be detected which moves fastwithout blur or distortion in an image.

Further, it is another object to provide a semiconductor deviceincluding a photosensor capable of imaging with high resolution with lowpower consumption.

An embodiment of the present invention relates to a semiconductor devicewhich includes a photosensor having a photodiode, a first transistor,and a second transistor. The photodiode has a function of generating anelectric signal in accordance with the intensity of light. The firsttransistor has a function of storing charge in a gate of the firsttransistor. The second transistor has a function of transferring theelectric signal generated by the photodiode to the gate of the firsttransistor. The second transistor has a function of holding the chargestored in the gate of the first transistor.

In the above structure, the first transistor has a back gate. In thefirst transistor, the threshold voltage can be changed by changing thepotential of the back gate.

In the above structure, the first transistor has a function ofconverting the charge stored in the gate into an output signal. Thecharge stored in the gate of the first transistor is converted into anoutput signal and the output signal is read, whereby an electric signalin accordance with the intensity of light can be output. Reading of thecharge stored in the gate of the first transistor is performed aplurality of times while the potential of the back gate of the firsttransistor is changed with the charge stored in the gate held.Specifically, the potential of the back gate of the first transistor isset at a first potential and the charge stored in the gate of the firsttransistor is converted into a first output signal and the first outputsignal is read. Then, the potential of the back gate of the firsttransistor is set at a second potential and the charge stored in thegate of the first transistor is converted into a second output signaland the second output signal is read. In the case where reading isperformed three times or more, the above-described operation may beperformed repeatedly. In this manner, reading of the charge stored inthe gate of the first transistor can be performed a plurality of timeswhile the potential of the back gate of the first transistor is changed.

Thus, even when the intensity of light is high, an electric signal inaccordance with the intensity of light can be output. Further, even whenthe intensity of light is low, an electric signal in accordance with theintensity of light can be output.

In the above structure, a channel formation region of at least thesecond transistor can be formed using an oxide semiconductor layer. Atransistor using an oxide semiconductor has an electric characteristicof extremely low off current when compared with a transistor usingsilicon or the like.

Therefore, by using an oxide semiconductor layer in the channelformation region of the second transistor, the charge stored in the gateof the first transistor can be held for a long time. Accordingly, thecharge stored in the gate of the first transistor can be held almostconstant in a period during which reading of the charge stored in thegate of the first transistor is performed a plurality of times.

In the above structure, the semiconductor device includes a thirdtransistor. The third transistor has a function of controlling readingof the output signal.

In the above structure, the semiconductor device includes a fourthtransistor. The fourth transistor has a function of controlling thepotential of a signal line which is used for reading the output signal.Specifically, the fourth transistor has a function of setting thepotential of the signal line at a reference potential.

According to an embodiment of the present invention, a semiconductordevice including a photosensor capable of outputting an electric signalin accordance with a wide range of intensity of light can be provided.That is, it is possible to accurately convert light, regardless of theintensity thereof, into an electric signal. Therefore, a semiconductordevice can be provided which includes a photosensor capable of realizingan imaging function with high resolution and applicability to a widerange of intensity of light in low cost.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating an example of a structure of asemiconductor device;

FIG. 2 is a diagram illustrating an example of a structure of a displaydevice;

FIG. 3 is a diagram illustrating an example of a circuit structure of apixel included in a display device;

FIG. 4 is an example of a timing chart of a photosensor;

FIG. 5 is an example of a timing chart of a photosensor;

FIG. 6 is an example of a timing chart of a photosensor;

FIGS. 7A to 7D are diagrams illustrating examples of a circuit structureof a photosensor;

FIGS. 8A to 8D are diagrams illustrating an example of a manufacturingprocess of a transistor included in a semiconductor device;

FIG. 9 is a diagram illustrating an example of a structure of atransistor included in a semiconductor device; and

FIG. 10 is a graph illustrating an example of a V_(g)-I_(d)characteristic of the transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described in detail with reference tothe accompanying drawings. However, since embodiments described belowcan be embodied in many different modes, it is easily understood bythose skilled in the art that the mode and the detail can be variouslychanged without departing from the scope of the present invention.Therefore, the present invention is not interpreted as being limited tothe description of the embodiments below. In the drawings for explainingthe embodiments, the same part or part having a similar function aredenoted by the same reference numerals, and description of such part isnot repeated.

Embodiment 1

In this embodiment, an example of a semiconductor device which is anembodiment of the disclosed invention is described with reference toFIG. 1.

An example of a circuit structure of a photosensor 106 included in thesemiconductor device is illustrated in FIG. 1. An example of a structureof a precharge circuit 200 which is electrically connected to thephotosensor 106 is also illustrated.

The photosensor 106 includes a photodiode 204, a transistor 205, atransistor 206, and a transistor 207.

In the photosensor 106, one electrode of the photodiode 204 iselectrically connected to a photodiode reset signal line 210, and theother electrode of the photodiode 204 is electrically connected to oneof a source and a drain of the transistor 207. One of a source and adrain of the transistor 205 is electrically connected to a photosensorreference signal line 213, and the other of the source and the drain ofthe transistor 205 is electrically connected to one of a source and adrain of the transistor 206. A gate of the transistor 206 iselectrically connected to a gate signal line 211, and the other of thesource and the drain of the transistor 206 is electrically connected toa photosensor output signal line 214. A gate of the transistor 207 iselectrically connected to a gate signal line 209. The other of thesource and the drain of the transistor 207 is electrically connected toa gate of the transistor 205 through a gate signal line 215.

The transistor 205 has a back gate. The back gate is electricallyconnected to a back gate signal line 218. By changing a potentialapplied to the back gate signal line 218, the potential of the back gateof the transistor 205 can be changed. By changing the potential of theback gate, the threshold voltage of the transistor 205 can be changed.The transistor 205 has a structure in which the gate, a gate insulatinglayer, a semiconductor layer including a channel formation region, aninsulating film, and the back gate are stacked. The insulating filmfunctions as a gate insulating layer on the back gate side. The gate andthe back gate are positioned so that the channel formation region isinterposed therebetween. The back gate can be formed using a conductivefilm similarly to the gate.

The gate signal line 209, the photodiode reset signal line 210, and thegate signal line 211 are electrically connected to a photosensor drivercircuit. The photosensor driver circuit has a function of performing areset operation, an accumulation operation, and a reading operation,which are described below, on the photosensor 106 arranged in aspecified row.

The photosensor output signal line 214, the photosensor reference signalline 213, and the back gate signal line 218 are electrically connectedto a photosensor reading circuit. The photosensor reading circuit has afunction of reading an output signal from the photosensor 106 in aselected row.

Note that the photosensor reading circuit can have a structure in whichan output from the photosensor which is an analog signal is extracted asan analog signal to the outside by an OP amplifier; or a structure inwhich the output is converted into a digital signal by an A/D convertercircuit and then extracted to the outside.

As the photodiode 204, a PN diode, a PIN diode, a Schottky diode, or anavalanche diode can be used. In the case where the PN diode or the PINdiode is used, a structure in which semiconductors having thecorresponding conductivity type (p-type conductivity and n-typeconductivity, or p-type conductivity, i-type conductivity, and n-typeconductivity) are stacked can be employed. Alternatively, a structure inwhich semiconductors each having a conductivity type are positioned onthe coplanar surface can be used. A semiconductor contained in thephotodiode 204 can be an amorphous semiconductor, a microcrystallinesemiconductor, a polycrystalline semiconductor, a single crystalsemiconductor, or the like. The photodiode has a function of generatingan electric signal in accordance with the intensity of light. Lightwhich is received by the photodiode is light reflected by an object orlight emitted from an object. As a light source of the light reflectedby the object, a lighting device included in the semiconductor device orexternal light can be used.

The transistor 207 has a function of controlling the accumulationoperation performed on the photosensor. That is, the transistor 207 in aconduction state has a function of transferring the electric signalgenerated by the photodiode 204 to the gate of the transistor 205.Therefore, a transistor with high mobility is desirably used as thetransistor 207. In addition, the transistor 207 in a non-conductionstate has a function of holding charge stored (accumulated) in the gateof the transistor 205. Thus, a transistor with extremely low off currentis desirably used as the transistor 207.

Therefore, as a semiconductor contained in a channel formation region ofthe transistor 207, an oxide semiconductor with extremely low offcurrent and comparatively high mobility is desirably used. A transistorusing an oxide semiconductor has an electric characteristic of extremelylow off current when compared with a transistor using silicon or thelike. A transistor using an oxide semiconductor has an electriccharacteristic of higher mobility than a transistor using amorphoussilicon.

The transistor 205 has a function of storing (accumulating) charge inthe gate. By converting the charge stored in the gate into an outputsignal and reading the output signal from the photosensor output signalline 214, the electric signal generated by the photodiode 204 can beread as the output signal. Reading of the charge stored in the gate ofthe transistor 205 is performed a plurality of times while the potentialof the back gate of the transistor 205 is changed with the charge storedin the gate of the transistor 205 held.

Accordingly, the photosensor 106 capable of outputting an electricsignal in accordance with a wide range of intensity of light can beprovided. That is, it is possible to accurately convert light,regardless of the intensity thereof, into an electric signal.

In order to perform the above-described reading at high speed, atransistor with high mobility is desirably used as the transistor 205.

The transistor 206 has a function of controlling reading of the outputsignal from the photosensor 106. Specifically, the transistor 206 has afunction of transferring the output signal from the photosensor 106 tothe photosensor output signal line 214. In order to perform transferringof the output signal at high speed, i.e., in order to perform reading ofthe output signal from the photosensor 106 at high speed, it isdesirable that a transistor with high mobility be used as the transistor206.

On the other hand, during a reading period for another pixel, it isnecessary to prevent an unnecessary potential from being output to thephotosensor output signal line 214. Therefore, it is desirable that atransistor with low off current be used as one or both of the transistor205 and the transistor 206.

Thus, in the case where high-speed reading is prioritized, a singlecrystal semiconductor, a polycrystalline semiconductor, or the like isdesirably used as a semiconductor contained in a channel formationregion of the transistors 205 and 206. Further, it is desirable to use amaterial (e.g., silicon) whose crystallinity is easily improved.

Moreover, in the case of prioritizing preventing an unnecessarypotential from being output, an oxide semiconductor with extremely lowoff current and comparatively high mobility is desirably used as asemiconductor contained in the channel formation region of one or bothof the transistor 205 and the transistor 206.

As described above, semiconductor materials used for the transistor 205and the transistor 206 can be selected depending on the characteristicsneeded for the photosensor 106.

Next, the precharge circuit 200 is described. The precharge circuit 200illustrated in FIG. 1 is used for pixels per column. The prechargecircuit 200 used for the pixels per column includes a transistor 216 anda precharge signal line 217. A gate of the transistor 216 iselectrically connected to the precharge signal line 217; one of a sourceand a drain of the transistor 216 is electrically connected to a signalline to which a predetermined potential is supplied; and the other ofthe source and the drain of the transistor 216 is electrically connectedto the photosensor output signal line 214. Note that an OP amplifier oran A/D converter circuit can be connected to a subsequent stage of theprecharge circuit 200.

In the precharge circuit 200, before the operation of the photosensor inthe pixel, the potential of the photosensor output signal line 214 isset at a reference potential. For example, when a high potential isapplied to the precharge signal line 217, the transistor 216 is turnedon and the photosensor output signal line 214 can be set at thereference potential (the high potential here). Note that it is effectiveto provide a storage capacitor for the photosensor output signal line214 so that the potential of the photosensor output signal line 214 isstabilized. Note that the reference potential can be set at a lowpotential.

According to this embodiment, a low-cost semiconductor device capable ofrealizing an imaging function with high resolution and applicability toa wide range of intensity of light can be provided.

Such a semiconductor device including a photosensor can be used in anelectronic appliance such as a scanner or a digital still camera. Inaddition, the semiconductor device including the photosensor can be usedin a display device having a touch panel function.

This embodiment can be implemented in combination with any of otherembodiments and an example as appropriate.

Embodiment 2

In this embodiment, an example of a semiconductor device which is anembodiment of the disclosed invention is described with reference toFIG. 2 and FIG. 3. An example in which the semiconductor device is adisplay device is described in this embodiment.

An example of a structure of a display device is illustrated in FIG. 2.A display device 100 includes a pixel circuit 101, a display elementcontrol circuit 102, and a photosensor control circuit 103. The pixelcircuit 101 includes a plurality of pixels 104 arranged in a matrix in arow direction and a column direction. Each of the pixels 104 includes adisplay element 105 and a photosensor 106. The photosensor 106 is notnecessarily provided in each of the pixels 104, and may be provided inevery two or more pixels. For example, a structure in which onephotosensor is provided per two pixels may be employed. Alternatively,the photosensor may be provided outside the pixel 104.

The display element control circuit 102 is a circuit controlling thedisplay element 105, and includes a display element driver circuit 107from which a signal such as a video data is input to the display element105 through a signal line (also referred to as a video-data signal lineor a source signal line); and a display element driver circuit 108 fromwhich a signal is input to the display element 105 through a scan line(also referred to as a gate signal line).

The photosensor control circuit 103 is a circuit controlling thephotosensor 106, and includes a photosensor reading circuit 109 on thesignal line side and a photosensor driver circuit 110 on the scan lineside.

In FIG. 3, an example of a circuit structure of the pixel 104 isillustrated. An example of a structure of the precharge circuit 200which is electrically connected to the pixel 104 is also illustrated.The precharge circuit 200 is included in the photosensor reading circuit109 which is illustrated in FIG. 2.

The pixel 104 includes the display element 105 and the photosensor 106.The display element 105 includes a transistor 201, a storage capacitor202, and a liquid crystal element 203.

In the display element 105, a gate of the transistor 201 is electricallyconnected to a gate signal line 208, one of a source and a drain of thetransistor 201 is electrically connected to a video data signal line212, and the other of the source and the drain of the transistor 201 iselectrically connected to one electrode of the storage capacitor 202 andone electrode of the liquid crystal element 203. The other electrode ofthe storage capacitor 202 and the other electrode of the liquid crystalelement 203 are electrically connected to a common wiring supplied witha predetermined potential. The liquid crystal element 203 is an elementincluding a pair of electrodes and a liquid crystal layer interposedbetween the pair of electrodes.

The transistor 201 has a function of controlling injection or ejectionof charge into or from the liquid crystal element 203 and the storagecapacitor 202. For example, when a high potential is applied to the gatesignal line 208, the transistor 201 is turned on and the potential ofthe video data signal line 212 is applied to the liquid crystal element203 and the storage capacitor 202. The contrast (gray scale) of lightpassing through the liquid crystal element 203 is made due to voltageapplication to the liquid crystal element 203, whereby image display isrealized. The storage capacitor 202 has a function of maintainingvoltage applied to the liquid crystal element 203. The display device100 which includes the liquid crystal element 203 can be a transmissivedisplay device, a reflective display device, or a semi-transmissivedisplay device.

The video data signal line 212 is electrically connected to the displayelement driver circuit 107 which is illustrated in FIG. 2. The displayelement driver circuit 107 is a circuit which supplies a signal to thedisplay element 105 through the video data signal line 212. The gatesignal line 208 is electrically connected to the display element drivercircuit 108 which is illustrated in FIG. 2. The display element drivercircuit 108 is a circuit which supplies a signal to the display element105 through the gate signal line 208. For example, the display elementdriver circuit 108 has a function of supplying a signal which selects adisplay element included in a pixel arranged in a specified row. Thedisplay element driver circuit 107 has a function of supplying a signalwhich supplies appropriate potentials to a display element included in apixel in a selected row.

As a semiconductor contained in a channel formation region of thetransistor 201, an amorphous semiconductor, a microcrystallinesemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,display quality can be improved by using an oxide semiconductor toobtain a transistor with extremely low off current.

Although the display element 105 described here includes the liquidcrystal element, the display element 105 may include another elementsuch as a light emitting element. The light emitting element is anelement whose luminance is controlled with current or voltage, andspecific examples thereof are a light emitting diode and an OLED(organic light emitting diode).

The photosensor 106 includes the photodiode 204, the transistor 205, thetransistor 206, and the transistor 207.

In the photosensor 106, one electrode of the photodiode 204 iselectrically connected to the photodiode reset signal line 210, and theother electrode of the photodiode 204 is electrically connected to oneof the source and the drain of the transistor 207. One of the source andthe drain of the transistor 205 is electrically connected to thephotosensor reference signal line 213, and the other of the source andthe drain of the transistor 205 is electrically connected to one of thesource and the drain of the transistor 206. The gate of the transistor206 is electrically connected to the gate signal line 211, and the otherof the source and the drain of the transistor 206 is electricallyconnected to the photosensor output signal line 214. The gate of thetransistor 207 is electrically connected to the gate signal line 209.The other of the source and the drain of the transistor 207 iselectrically connected to the gate of the transistor 205 through thegate signal line 215.

The transistor 205 has the back gate. The back gate is electricallyconnected to the back gate signal line 218. By changing a potentialapplied to the back gate signal line 218, the potential of the back gateof the transistor 205 can be changed. By changing the potential of theback gate, the threshold voltage of the transistor 205 can be changed.The transistor 205 has the structure in which the gate, the gateinsulating layer, the semiconductor layer including the channelformation region, the insulating film, and the back gate are stacked.The insulating film functions as the gate insulating layer on the backgate side. The gate and the back gate are positioned so that the channelformation region is interposed therebetween. The back gate can be formedusing a conductive film similarly to the gate.

The gate signal line 209, the photodiode reset signal line 210, and thegate signal line 211 are electrically connected to the photosensordriver circuit 110 which is illustrated in FIG. 2. The photosensordriver circuit 110 has a function of performing the reset operation, theaccumulation operation, and the reading operation, which are describedbelow, on the photosensor 106 included in a pixel arranged in aspecified row.

The photosensor output signal line 214, the photosensor reference signalline 213, and the back gate signal line 218 are electrically connectedto the photosensor reading circuit 109 which is illustrated in FIG. 2.The photosensor reading circuit 109 has a function of reading an outputsignal from the photosensor 106 included in a pixel in a selected row.

Note that the photosensor reading circuit 109 can have a structure inwhich an output from the photosensor which is an analog signal isextracted as an analog signal to the outside by an OP amplifier; or astructure in which the output is converted into a digital signal by anA/D converter circuit and then extracted to the outside.

As the photodiode 204, a PN diode, a PIN diode, a Schottky diode, or anavalanche diode can be used. In the case where the PN diode or the PINdiode is used, a structure in which semiconductors having thecorresponding conductivity type (p-type conductivity and n-typeconductivity, or p-type conductivity, i-type conductivity, and n-typeconductivity) are stacked can be employed. Alternatively, a structure inwhich semiconductors each having a conductivity type are positioned onthe coplanar surface can be used. A semiconductor contained in thephotodiode 204 can be an amorphous semiconductor, a microcrystallinesemiconductor, a polycrystalline semiconductor, a single crystalsemiconductor, or the like. The photodiode has a function of generatingan electric signal in accordance with the intensity of light. In thedisplay device 100, light which is received by the photodiode is lightreflected by an object or light emitted from an object. As a lightsource of the light reflected by the object, a lighting device includedin the display device or external light can be used. In the case where alight emitting element is used as the display element 105 included inthe display device, light emitted from the light emitting element can beutilized as the light source of the light reflected by the object.

The transistor 207 has a function of controlling the accumulationoperation performed on the photosensor. That is, the transistor 207 in aconduction state has a function of transferring the electric signalgenerated by the photodiode 204 to the gate of the transistor 205.Therefore, a transistor with high mobility is desirably used as thetransistor 207. In addition, the transistor 207 in a non-conductionstate has a function of holding charge stored (accumulated) in the gateof the transistor 205. Thus, a transistor with extremely low off currentis desirably used as the transistor 207.

Therefore, as a semiconductor contained in the channel formation regionof the transistor 207, an oxide semiconductor with extremely low offcurrent and comparatively high mobility is desirably used. A transistorusing an oxide semiconductor has an electric characteristic of extremelylow off current when compared with a transistor using silicon or thelike. A transistor using an oxide semiconductor has an electriccharacteristic of higher mobility than a transistor using amorphoussilicon.

Moreover, as described in Embodiment 1, semiconductor materials used forthe transistor 205 and the transistor 206 can be selected depending onthe characteristics needed for the photosensor 106.

Next, the precharge circuit 200 is described. The precharge circuit 200illustrated in FIG. 3 is used for pixels per column. The prechargecircuit 200 used for the pixels per column includes the transistor 216and the precharge signal line 217. The gate of the transistor 216 iselectrically connected to the precharge signal line 217; one of thesource and the drain of the transistor 216 is electrically connected toa signal line to which a predetermined potential is supplied; and theother of the source and the drain of the transistor 216 is electricallyconnected to the photosensor output signal line 214. Note that an OPamplifier or an A/D converter circuit can be connected to a subsequentstage of the precharge circuit 200.

In the precharge circuit 200, before the operation of the photosensor inthe pixel, the potential of the photosensor output signal line 214 isset at a reference potential. For example, when a high potential isapplied to the precharge signal line 217, the transistor 216 is turnedon and the photosensor output signal line 214 can be set at thereference potential (the high potential here). Note that it is effectiveto provide a storage capacitor for the photosensor output signal line214 so that the potential of the photosensor output signal line 214 isstabilized. Note that the reference potential can be set at a lowpotential.

Although the display device including the photosensor is described inthis embodiment, this embodiment can be easily applied to asemiconductor device including a photosensor, which does not have adisplay function. That is, the semiconductor device can be formed byremoving the circuits necessary for display, specifically the displayelement control circuit 102 and the display element 105, from thedisplay device 100 in this embodiment. As a semiconductor deviceincluding a photosensor, an imaging device which is used in anelectronic appliance such as a scanner or a digital still camera can begiven.

According to this embodiment, a low-cost display device or a low-costsemiconductor device which is capable of realizing an imaging functionwith high resolution and applicability to a wide range of intensity oflight can be provided.

This embodiment can be implemented in combination with any of otherembodiments and an example as appropriate.

Embodiment 3

In this embodiment, an example of the operation of the semiconductordevice which is illustrated in FIG. 1 or an example of the operation ofthe display device which is illustrated in FIG. 2 and FIG. 3 isdescribed with reference to FIG. 4. FIG. 4 is an example of a timingchart related to the reading operation on the photosensor 106illustrated in FIG. 1 or FIG. 2 and FIG. 3.

In FIG. 4, a signal 301, a signal 302, a signal 303, a signal 304, asignal 305, a signal 306, and a signal 307 correspond to the potentialsof the photodiode reset signal line 210, the gate signal line 209, thegate signal line 211, the gate signal line 215, the photosensor outputsignal line 214, the precharge signal line 217, and the back gate signalline 218 in FIG. 1 or FIG. 3, respectively. Note that the photosensorreference signal line 213 is set at a low potential.

The timing chart of FIG. 4 includes a reset period during which thereset operation is performed, an accumulating period during which thecharge accumulation operation is performed, and a reading period duringwhich the reading operation is performed. A period from time A to time Bcorresponds to the reset period. A period from the time B to time Ccorresponds to the accumulation period. A period from time D to time Ecorresponds to a first reading period and a period from time G to time Hcorresponds to a second reading period.

Hereinafter, a high potential is expressed as “H” and a low potential isexpressed as “L”. In addition, hereinafter, an example in which atransistor is turned on when a gate of the transistor is supplied with ahigh potential (“H”) signal. Further, hereinafter, description is madewith the transistor 205 being a transistor whose threshold voltage isdecreased (increased) by increasing (decreasing) the potential of a backgate.

At the time A, the potential of the photodiode reset signal line 210(the signal 301) is set at “H” and the potential of the gate signal line209 (the signal 302) is set at “H” (the reset operation is started);then, the photodiode 204 and the transistor 207 are brought intoelectrical conduction and the potential of the gate signal line 215 (thesignal 304) becomes “H”. Thus, charge corresponding to a high potential(“H”) is stored in the gate signal line 215.

When the potential of the precharge signal line 217 (the signal 306) is“H”, the potential of the photosensor output signal line 214 (the signal305) is precharged to “H”. The potential of the back gate signal line218 (the signal 307) is 0 V and the threshold voltage of the transistor205 is around 0 V at this time.

At the time B, the potential of the photodiode reset signal line 210(the signal 301) is set at “L” and the potential of the gate signal line209 (the signal 302) is kept at “H” (the reset operation is completedand the accumulation operation is started); then, the potential of thegate signal line 215 (the signal 304) starts to decrease because of theleakage current of the photodiode 204. When light is received by thephotodiode 204, leakage current (also called photocurrent) is increased;therefore, the potential of the gate signal line 215 (the signal 304) ischanged in accordance with the intensity of the received light(specifically the light reflected by the object). In other words, theamount of the charge in the gate signal line 215 is changed inaccordance with the photocurrent generated in the photodiode 204. Thus,the amount of the charge stored in the gate of the transistor 205 ischanged and channel resistance between the source and the drain of thetransistor 205 is changed. When the photocurrent generated in thisphotodiode 204 is regarded as an electric signal, it means that theamount of the charge in the gate signal line 215 is changed inaccordance with the electric signal generated in this photodiode 204.

At the time C, the potential of the gate signal line 209 (the signal302) is set at “L” (the accumulation operation is completed), wherebythe transistor 207 is turned off and the potential of the gate signalline 215 (the signal 304) becomes constant. That is, the amount of thecharge stored (accumulated) in the gate signal line 215 becomes constantand the amount of the charge stored (accumulated) in the gate of thetransistor 205 becomes constant. The potential (the amount of thecharge) of the gate signal line 215 is determined depending on theamount of the photocurrent generated in the photodiode during theaccumulation operation. In other words, the potential (the amount of thecharge) of the gate signal line 215 is changed in accordance with theintensity of the light which is received by the photodiode.

The transistor 207 is a transistor with extremely low off current whichuses an oxide semiconductor layer in its channel formation region.Therefore, until the subsequent reading operation is performed, theamount of the stored charge can be kept constant. As described above,the transistor 207 has a function of controlling the accumulationoperation in which charge is stored (accumulated) in the gate of thetransistor 205.

Note that the potential (the amount of the charge) of the gate signalline 215 is changed due to parasitic capacitance between the gate signalline 209 and the gate signal line 215 at the time of setting thepotential of the gate signal line 209 (the signal 302) at “L”. When theamount of change in the potential (the amount of the charge) due to theparasitic capacitance is large, reading cannot be performed accurately.In order that the amount of change in the potential (the amount of thecharge) due to the parasitic capacitance be reduced, it is effective toreduce capacitance between the gate and the source (or between the gateand the drain) of the transistor 207, to increase the gate capacitanceof the transistor 205, to provide the gate signal line 215 with astorage capacitor, or the like. Note that in FIG. 4, these methods areapplied, and change in the potential (the amount of the charge) due tothe parasitic capacitance can be thus ignored.

The transistor 205 is turned on or off depending on the potential of thegate signal line 215 (the signal 304). In the case where the intensityof the light which is received by the photodiode 204 is low, decrease inpotential of the gate signal line 215 (the signal 304) from thepotential “H” is small. Therefore, the transistor 205 is turned on andthe channel resistance between the source and the drain is lowered. Incontrast, in the case where the intensity of the light which is receivedby the photodiode 204 is high, decrease in potential of the gate signalline 215 (the signal 304) from the potential “H” is large. Thus, thetransistor 205 exists in an off state or in a conduction state withincreased channel resistance between the source and the drain. Here, thepotential of the gate signal line 215 (the signal 304) after theaccumulation operation (at the time C) is assumed to become a valuewhich is able to keep the transistor 205 in an on state.

At the time D, the potential of the gate signal line 211 (the signal303) is set at “H” (a first reading operation is started); then, thetransistor 206 is turned on and the photosensor reference signal line213 and the photosensor output signal line 214 are brought intoelectrical conduction through the transistor 205 and the transistor 206.Since the photosensor reference signal line 213 is set at the lowpotential, the potential of the photosensor output signal line 214 (thesignal 305) decreases. Note that before the time D, the potential of theprecharge signal line 217 (the signal 306) is set at “L” so that theprecharge of the photosensor output signal line 214 is completed. Here,the rate at which the potential of the photosensor output signal line214 (the signal 305) decreases depends on the channel resistance betweenthe source and the drain of the transistor 205; namely, the rate ischanged depending on the intensity of the light which is received by thephotodiode 204 during the accumulation operation.

At the time E, the potential of the gate signal line 211 (the signal303) is set at “L” (the first reading operation is completed); then, thetransistor 206 is turned off and the potential of the photosensor outputsignal line 214 (the signal 305) becomes constant. The potential of thephotosensor output signal line 214 here depends on the intensity of thelight which is received by the photodiode 204. Thus, the intensity ofthe light which is received by the photodiode 204 during theaccumulation operation can be determined by detecting the potential ofthe photosensor output signal line 214.

Here, in the case where the intensity of the light which is received bythe photodiode 204 is low, the transistor 205 is turned on and thechannel resistance between the source and the drain is low. Thus,decrease in potential of the photosensor output signal line 214 (thesignal 305) from the potential “H” is large, and its potential becomesclose to the potential of the photosensor reference signal line 213. Inthis case, the light cannot be distinguished from weaker light.Therefore, in order to distinguish the light from weaker light, it ispossible to extend a voltage range between the potential of thephotosensor reference signal line 213 and the reference potential of thephotosensor output signal line 214 at the time of precharge; however, bythis method, an A/D converter circuit which can operate in a widevoltage range is needed and the manufacturing cost of the semiconductordevice or the display device is increased. In view of the above, adriving method described below is employed.

The potential of the back gate signal line 218 (the signal 307) is setat a negative potential before time F. At this time, the thresholdvoltage of the transistor 205 is higher than 0 V. At the time F, thepotential of the precharge signal line 217 (the signal 306) is “H” andthe potential of the photosensor output signal line 214 (the signal 305)is precharged to “H”.

At the time G, the potential of the gate signal line 211 (the signal303) is set at “H” (a second reading operation is started); then, thetransistor 206 is turned on and the photosensor reference signal line213 and the photosensor output signal line 214 are brought intoelectrical conduction through the transistor 205 and the transistor 206.Then, the potential of the photosensor output signal line 214 (thesignal 305) decreases. Note that before the time G, the potential of theprecharge signal line 217 (the signal 306) is set at “L” so that theprecharge of the photosensor output signal line 214 is completed. Here,the rate at which the potential of the photosensor output signal line214 (the signal 305) decreases depends on the channel resistance betweenthe source and the drain of the transistor 205; namely, the rate dependson the intensity of the light which is received by the photodiode 204during the accumulation operation. However, since the threshold voltageof the transistor 205 is higher than that at the time of the firstreading operation, the rate at which the potential of the photosensoroutput signal line 214 (the signal 305) decreases is lowered.

At the time H, the potential of the gate signal line 211 (the signal303) is set at “L” (the second reading operation is completed); then,the transistor 206 is turned off and the potential of the photosensoroutput signal line 214 (the signal 305) becomes constant. The potentialof the photosensor output signal line 214 here depends on the intensityof the light which is received by the photodiode 204. Thus, theintensity of the light which is received by the photodiode 204 duringthe accumulation operation can be determined by detecting the potentialof the photosensor output signal line 214. In this manner, even in thecase where the intensity of the light is low, the intensity of the lightwhich is received by the photodiode 204 can be detected with the use ofa low-cost A/D converter circuit.

The case where the intensity of the light which is received by thephotodiode is low (i.e., the light is weak) is described above;similarly, the driving method can be applied to the case where theintensity of the light which received by the photodiode is high (i.e.,the light is strong). In the case where the light is strong, thepotential of the photosensor output signal line 214 is almost the sameas the reference potential at the time of precharge and is difficult tobe detected. Therefore, as a third reading operation, the potential ofthe back gate signal line 218 is set at a positive potential so that thethreshold voltage of the transistor 205 is lower than 0 V. Accordingly,the rate at which the potential of the photosensor output signal line214 (the signal 305) decreases is increased and the potential of thephotosensor output signal line 214 is more easily detected.

Further, in order that both the case where the light which is receivedby the photodiode is strong and the case where the light is weak bedetected, it is effective to perform the above-described first to thirdreading operations repeatedly. That is, the potential of the back gateof the first transistor is set at a first potential (0 V here) and thecharge stored in the gate of the first transistor is converted into afirst output signal and the first output signal is read. Then, thepotential of the back gate of the first transistor is set at a secondpotential (a negative potential here) and the charge stored in the gateof the first transistor is converted into a second output signal and thesecond output signal is read. After that, the potential of the back gateof the first transistor is set at a third potential (a positivepotential here) and the charge stored in the gate of the firsttransistor is converted into a third output signal and the third outputsignal is read. In addition, by changing the potential of the back gatesignal line 218 at the time of the second and third reading operationsby a narrower voltage width and reading is sequentially performed,detection with high resolution can be performed on light in a widerrange of intensity. In other words, by employing the above-describedstructure, the photosensor 106 can be provided which is capable ofaccurately converting light, regardless of the intensity thereof, intoan electric signal and outputting an electric signal in accordance witha wide range of intensity of light.

In order to realize the aforementioned driving method, the potential ofthe gate signal line 215 in each photosensor needs to be held constanteven after the accumulation operation is completed. Therefore, asdescribed with reference to FIG. 1 or FIG. 3, a structure is effectivein which the transistor 207 is formed using the oxide semiconductorlayer so as to have extremely low off current.

In the above manner, the operation of individual photosensors isrealized by repeating the reset operation, the accumulation operation,and the reading operation. By employing this driving method in all thepixels, imaging can be performed. More specifically, imaging can beperformed by repeating the reset operation, the accumulation operation,and the reading operation per row.

According to this embodiment, a low-cost semiconductor device or alow-cost display device which is capable of realizing an imagingfunction with high resolution dealing with a wide range of intensity oflight can be provided.

This embodiment can be implemented in combination with any of otherembodiments and an example as appropriate.

Embodiment 4

In this embodiment, an example of a driving method of a semiconductordevice including a plurality of photosensors is described.

First, a driving method illustrated in the timing chart of FIG. 5 isdescribed. In FIG. 5, a signal 801, a signal 802, and a signal 803correspond to the photodiode reset signal lines 210 in photosensors inthe first row, the second row, and the third row, respectively. A signal804, a signal 805, and a signal 806 correspond to the gate signal lines209 in the photosensors in the first row, the second row, and the thirdrow, respectively. A signal 807, a signal 808, and a signal 809correspond to the gate signal lines 211 in the photosensors in the firstrow, the second row, and the third row, respectively. A period 810 is aperiod during which imaging is performed once. A period 811 is a periodduring which the photosensor in the second row performs the resetoperation; a period 812 is a period during which the photosensor in thesecond row performs the accumulation operation; and a period 813 is aperiod during which the photosensor in the second row performs thereading operation. By thus sequentially driving the photosensors indifferent rows, an image can be taken.

Here, the accumulation operations in the photosensors of different rowshave a time lag between one another. That is, imaging in thephotosensors in all rows is not performed simultaneously, leading toblurring of the image taken. In particular, an image of an object to bedetected which moves fast is likely to be taken to have a distortedshape: if the object to be detected moves in a direction from the firstrow to the third row, an enlarged image is taken as if it leaves a trailbehind it; and if the object to be detected moves in the oppositedirection, a reduced image is taken.

In order to prevent the time lag of the accumulation operations in thephotosensors in different rows, it is effective to reduce the intervalbetween the operations of the photosensors in different rows. In thatcase, however, the output signal from the photosensor needs to beobtained at very high speed with an OP amplifier or an AD convertercircuit, which causes an increase in power consumption. It is difficultto obtain the output signal from the photosensor with an OP amplifier oran AD converter circuit at very high speed particularly when an imagewith high resolution is obtained.

In view of the above, a driving method illustrated in the timing chartof FIG. 6 is proposed. In FIG. 6, a signal 501, a signal 502, and asignal 503 correspond to the photodiode reset signal lines 210 in thephotosensors in the first row, the second row, and the third row,respectively. A signal 504, a signal 505, and a signal 506 correspond tothe gate signal lines 209 in the photosensors in the first row, thesecond row, and the third row, respectively. A signal 507, a signal 508,and a signal 509 correspond to the gate signal lines 211 in thephotosensors in the first row, the second row, and the third row,respectively. A period 510 is a period during which imaging is performedonce. A period 511 is a period during which the reset operation isperformed in the photosensor in the second row (at the same time as inthe other rows), a period 512 is a period during which the accumulationoperation is performed in the photosensor in the second row (at the sametime as in the other rows), and a period 513 is a period during whichthe reading operation is performed in the photosensor in the second row.

FIG. 6 is different from FIG. 5 in that the reset operation and theaccumulation operation are each performed simultaneously in thephotosensors in all the rows, and after the accumulation operation, thereading operation is sequentially performed row by row withoutsynchronization with the accumulation operation. When the accumulationoperation is performed simultaneously, imaging in the photosensors ofall rows is performed simultaneously and an image of an object to bedetected can be easily taken with negligible blur even when the objectto be detected moves fast. Since the accumulation operation is performedat the same time, a driver circuit can be provided in common for thephotodiode reset signal lines 210 of a plurality of photosensors. Adriver circuit can also be provided in common for the gate signal lines209 of a plurality of photosensors. Such driver circuits provided incommon are effective in reducing the number of peripheral circuits orreducing power consumption. In addition, the reading operationsequentially performed row by row makes it possible to decrease theoperation rate of an OP amplifier or an A/D converter circuit when theoutput signal from the photosensor is obtained. The total time for thereading operation is preferably longer than the time for theaccumulation operation, which is particularly effective in the case ofobtaining an image with high resolution.

In the timing charts of FIG. 5 and FIG. 6, the periods 810 and 510during which imaging is performed once includes a plurality of theperiods 813 and 513 during which the reading operation is performed.Although two periods 813 and 513 are included in FIG. 5 and FIG. 6,three or more periods 813 and 513 during which the reading operation isperformed are preferably included in the periods 810 and 510 so thatboth the case where the light is strong and the case where the light isweak are dealt with. A plurality of the reading operations is performedas illustrated in the timing charts of FIG. 5 and FIG. 6, in such amanner that the first reading operation is first performed row by row inall rows, the second reading operation is then performed row by row inall rows, and operation in this manner is repeated until the n-threading operation (n is an integer greater than or equal to 3) isperformed. Alternatively, the first to n-th reading operations areperformed in the first row first, the first to n-th reading operationsare then performed in the second row, and operation in this manner isrepeated until the first to n-th reading operations are performed in them-th row (m is an integer greater than or equal to 3).

Note that FIG. 5 and FIG. 6 illustrate the timing chart of the methodfor sequentially driving the photosensors row by row; it is alsoeffective to sequentially drive the photosensor only in specified rowsin order to obtain an image in a specified region. Thus, a desired imagecan be obtained while the operation and power consumption of the OPamplifier or the A/D converter circuit are reduced.

In order to realize the aforementioned driving method, the potential ofthe gate signal line 215 in each photosensor needs to be held constanteven after the accumulation operation is completed. Thus, the transistor207 is preferably formed using an oxide semiconductor so as to haveextremely low off current as described with reference to FIG. 1 or FIG.3.

In the aforementioned manner, it is possible to provide a low-powerconsumption display device or a low-power consumption semiconductordevice which allows a high-resolution image of an object to be detectedwith little blur to be taken even when the object to be detected movesfast.

This embodiment can be implemented in combination with any of otherembodiments and an example as appropriate.

Embodiment 5

In this embodiment, modified examples of the circuit structure of thephotosensor 106 in FIG. 1 or FIG. 3 are described.

FIG. 7A illustrates a structure in which the gate of the transistor 205in FIG. 1 or FIG. 3 is connected to a transistor 601 for controlling thereset operation of the photosensor. Specifically, one of a source and adrain of the transistor 601 is electrically connected to the photosensorreference signal line 213 and the other thereof is electricallyconnected to the gate of the transistor 205. One electrode of thephotodiode 204 is electrically connected to a wiring to which apredetermined potential (e.g., a ground potential) is applied.

The transistor 601 can be formed using an amorphous semiconductor, amicrocrystalline semiconductor, a polycrystalline semiconductor, anoxide semiconductor, a single crystal semiconductor, or the like. Inparticular, an oxide semiconductor is preferably used for the transistor601 so that off current of the transistor 601 is low and charge of thegate of the transistor 205 is prevented from being released through thetransistor 601 after the reset operation.

FIG. 7B illustrates a structure in which the transistor 205 and thetransistor 206 are connected to be opposite to those in FIG. 7A.Specifically, one of the source and the drain of the transistor 205 iselectrically connected to the photosensor output signal line 214, andone of the source and the drain of the transistor 206 is electricallyconnected to the photosensor reference signal line 213.

FIG. 7C illustrates a structure in which the transistor 206 is omittedfrom the structure in FIG. 7A. Specifically, one of the source and thedrain of the transistor 205 is electrically connected to the photosensorreference signal line 213 and the other thereof is electricallyconnected to the photosensor output signal line 214.

Note that in FIGS. 7A to 7C, one of the source and the drain of thetransistor 601 may be electrically connected to a wiring other than thephotosensor reference signal line 213.

In FIG. 7D, one of the source and the drain of the transistor 601 inFIG. 7C is electrically connected to the photosensor output signal line214 and the other thereof is electrically connected to the gate of thetransistor 205.

In FIGS. 7A to 7D, when the transistor 207 is formed using an oxidesemiconductor to reduce off current, the charge stored in the gate ofthe transistor 205 can be held constant.

In FIGS. 7A to 7D, by providing a back gate for the transistor 205, asemiconductor device capable of realizing an imaging function with highresolution and applicability to a wide range of intensity of light canbe provided.

In FIGS. 7A to 7D, connection of the two electrodes of the photodiode204 may be counterchanged depending on the circuit structure of thephotosensor.

This embodiment can be implemented in combination with any of otherembodiments and an example as appropriate.

Embodiment 6

In this embodiment, an example of a transistor included in asemiconductor device which is an embodiment of the disclosed inventionis described. Specifically, an example of a transistor in which achannel formation region is formed using an oxide semiconductor layersimilarly to the transistor 207 illustrated in FIG. 1 or FIG. 3 or thetransistor 201 illustrated in FIG. 3 is described.

<Transistor>

The transistor (e.g., the transistor 207 illustrated in FIG. 1 or FIG.3) is a transistor whose channel formation region is formed using anoxide semiconductor layer. The oxide semiconductor layer is highlypurified to become electrically i-type (intrinsic) or substantiallyi-type (intrinsic) by the following way: impurities such as hydrogen,moisture, a hydroxyl group, and a hydride, which can be donors and leadto change in electrical characteristics of the transistor, arethoroughly removed to decrease their concentration as much as possible;and oxygen which is a main component of the oxide semiconductor anddecreases in concentration through the step of removing the impuritiesis supplied. Note that the oxide semiconductor contained in the oxidesemiconductor layer has a band gap of 3.0 eV or larger.

Further, the highly purified oxide semiconductor has very few carriers(close to zero) and the carrier density is extremely low (for example,lower than 1×10¹²/cm³, preferably lower than 1×10¹¹/cm³). Accordingly,the off current of the transistor is extremely low. Therefore, in theaforementioned transistor, off current per micrometer of the channelwidth (w) at room temperature can be 1 aA/μm (1×10⁻¹⁸ A/μm) or lower,and further can be less than 100 zA/μm (1×10⁻¹⁹ A/μm). In general, in atransistor using amorphous silicon, the off current at room temperatureis 1×10⁻¹³ A/μm or larger. In addition, it can be considered that hotcarrier deterioration does not occur in the transistor using a highlypurified oxide semiconductor layer which is described above. Thus, theelectrical characteristics of the transistor is not affected by hotcarrier deterioration.

An oxide semiconductor layer which is highly purified by thoroughlyremoving hydrogen contained in the oxide semiconductor layer asdescribed above is used in a channel formation region of a transistor,whereby a transistor with extremely low off current can be obtained.That is, in circuit design, the oxide semiconductor layer can beregarded as an insulator when the transistor is in an off state (alsoreferred to as a non-conduction state). On the other hand, when thetransistor in which the oxide semiconductor is used in a channelformation region is in an on state (a conduction state), the transistoris expected to exhibit higher current supply capability than atransistor using amorphous silicon.

It is assumed that the off current of a transistor using alow-temperature polysilicon at room temperature is approximately 10000times as large as the off current of a transistor using an oxidesemiconductor. Therefore, with the transistor using an oxidesemiconductor, the charge holding period can be prolonged byapproximately 10000 times as long as that of the transistor usinglow-temperature polysilicon.

As described above, the transistor using a highly purified oxidesemiconductor layer in a channel formation region is capable of holdingcharge stored in a source or a drain of the transistor for a long time.

Therefore, by using an oxide semiconductor layer in the channelformation region of the transistor 207 illustrated in FIG. 1 or FIG. 3,the charge stored in the gate of the transistor 205 illustrated in FIG.1 or FIG. 3 can be held for a long time. Accordingly, the charge storedin the gate of the transistor 205 can be held almost constant in aperiod during which reading of the charge stored in the gate of thetransistor 205 is performed a plurality of times. Thus, by using theoxide semiconductor layer in the channel formation region of thetransistor 207, a semiconductor device which includes a photosensor witha novel circuit structure can be provided.

Further, by using the oxide semiconductor layer in the channel formationregion of the transistor 201 illustrated in FIG. 3, an image signalholding period of a pixel can be extended. Therefore, the size of acapacitor provided in a pixel can be reduced. Thus, the aperture ratioof the pixel can be high, and an image signal can be input to the pixelat high speed, for example. In addition, a rewriting interval of animage signal in still image display can be longer. For example, awriting interval of an image signal can be 10 seconds or longer, 30seconds or longer, or 1 minute or longer and shorter than 10 minutes.The longer the writing interval is, the more the power consumption canbe reduced.

Note that in this specification, a semiconductor with carrierconcentration lower than 1×10¹¹/cm³ is called an “intrinsic” (“i-type”)semiconductor, and a semiconductor with carrier concentration of1×10¹¹/cm³ or higher and lower than 1×10¹²/cm³ is called a“substantially intrinsic” (“substantially i-type”) semiconductor.

<Manufacturing Method of Transistor>

An example of a manufacturing method of a transistor whose channelformation region is formed using an oxide semiconductor layer isdescribed with reference to FIGS. 8A to 8D.

FIGS. 8A to 8D are cross-sectional views illustrating an example of astructure and a manufacturing process of a transistor whose channelformation region is formed using an oxide semiconductor layer. Atransistor 410 illustrated in FIG. 8D has an inverted staggeredstructure that is one of bottom gate structures. The transistor 410 isalso a channel etched transistor with a single gate structure.

However, the structure of the transistor is not limited to the abovedescription and may have a top gate structure. Alternatively, thetransistor may be a channel stopped transistor, and the transistor mayhave a multigate structure.

A process for manufacturing the transistor 410 over a substrate 400 isdescribed below with reference to FIGS. 8A to 8D.

First, a gate electrode layer 411 is formed over the substrate 400 whichhas an insulating surface (see FIG. 8A).

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to a heattreatment to be performed later. For example, a glass substrate can beused as the substrate 400 having an insulating surface. Further, anelement substrate in which an element is formed over a glass substrateor a single crystal substrate can be used as the substrate 400 having aninsulating surface. In the case where the element substrate is used, thesubstrate can have an insulating layer over the surface.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 411. The base film has afunction of preventing diffusion of impurity elements from the substrate400, and can be formed to have a single-layer structure or a stackedstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film. Inthis embodiment, a silicon nitride film is formed to a thickness of 100nm by a plasma CVD method and a silicon oxynitride film (a SiON film) isformed to a thickness of 150 nm by a plasma CVD method over the siliconnitride film.

Note that the base film is preferably formed so as to contain impuritiessuch as hydrogen and water as little as possible.

The gate electrode layer 411 can be formed in such a manner that aconductive layer is formed over the substrate 400 and is selectivelyetched by a first photolithography step.

The gate electrode layer 411 can be formed to have a single-layerstructure or a stacked structure using a metal such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium; an alloy which contains any of these metals as its maincomponent; or a nitride which contains any of these metal elements asits main component. In this embodiment, a tungsten film is formed to athickness of 100 nm by a sputtering method and is etched so as to formthe gate electrode layer 411.

Then, a gate insulating layer 402 is formed over the gate electrodelayer 411 (see FIG. 8A).

The gate insulating layer 402 can be formed with a single-layerstructure or a stacked structure using one or more of a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, a siliconnitride oxide layer, and an aluminum oxide layer by a plasma CVD method,a sputtering method, or the like. For example, a silicon oxynitridelayer may be formed using a deposition gas containing silane (SiH₄),oxygen, and nitrogen by a plasma CVD method. Furthermore, a high-kmaterial such as hafnium oxide (HfO_(x)) or tantalum oxide (TaO_(x)) canbe used for the gate insulating layer. The thickness of the gateinsulating layer 402 can be greater than or equal to 10 nm and less thanor equal to 500 nm, for example.

Here, as the gate insulating layer, a silicon oxynitride film is formedto a thickness of 100 nm by a high-density plasma CVD method using amicrowave (e.g., with a frequency of 2.45 GHz) over the gate electrodelayer 411. It is preferable to employ a high-density plasma CVD methodusing a microwave because the gate insulating layer 402 can be dense andhave high withstand voltage and high quality. When the oxidesemiconductor layer and the high-quality gate insulating layer 402 arein contact with each other, the interface state density can be reducedand interface properties can be favorable.

Note that the gate insulating layer 402 is preferably formed so as tocontain impurities such as hydrogen and water as little as possible.That is, the gate insulating layer 402 is preferably formed in such amanner that the concentration of the impurities such as hydrogen andwater contained is decreased as much as possible.

Next, an oxide semiconductor film 430 is formed over the gate insulatinglayer 402 (see FIG. 8A). The oxide semiconductor film 430 can be formedby a sputtering method. The thickness of the oxide semiconductor film430 can be set greater than or equal to 2 nm and less than or equal to200 nm.

Note that before the oxide semiconductor film 430 is formed by asputtering method, reverse sputtering in which an argon gas isintroduced and plasma is generated is preferably performed. Byperforming reverse sputtering, powdery substances (also referred to asparticles or dust) which are attached on a surface of the gateinsulating layer 402 can be removed. The reverse sputtering refers to amethod in which without application of voltage to the target side, an RFpower source is used for application of voltage to the substrate side sothat plasma is generated to modify a surface of the substrate. Note thatinstead of an argon atmosphere, a nitrogen atmosphere, a heliumatmosphere, an oxygen atmosphere, or the like may be used.

As the oxide semiconductor film 430, an In—Ga—Zn—O-based material, anIn—Sn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, an In—Zn—O-basedmaterial, a Sn—Zn—O-based material, an Al—Zn—O-based material, anIn—O-based material, a Sn—O-based material, or a Zn—O-based material canbe used. In addition, the above materials may contain SiO₂.

The oxide semiconductor film 430 can be formed by a sputtering method ina rare gas (typically argon) atmosphere, an oxygen atmosphere, or amixed atmosphere containing a rare gas (typically argon) and oxygen.

Here, an oxide semiconductor layer is formed to a thickness of 30 nm bya sputtering method using an In—Ga—Zn—O-based metal oxide target whichcontains In, Ga, and Zn. Note that a sputtering gas used contains Ar andO₂ and the substrate temperature is set to 200° C.

Note that the oxide semiconductor film 430 is preferably formed so as tocontain impurities such as hydrogen and water as little as possible.That is, the oxide semiconductor film 430 is preferably formed in such amanner that the concentration of the impurities such as hydrogen andwater contained is decreased as much as possible.

Next, the oxide semiconductor film 430 is selectively etched by a secondphotolithography step so that an island-shaped oxide semiconductor layer431 is formed (see FIG. 8B). For etching the oxide semiconductor film430, wet etching can be employed. Note that dry etching may also be usedwithout limitation to the wet etching.

Next, a first heat treatment is performed on the oxide semiconductorlayer 431. An excess amount of water, hydrogen, and the like that arecontained in the oxide semiconductor layer 431 can be removed by thefirst heat treatment. The temperature of the first heat treatment can behigher than or equal to 350° C. or higher and lower than the strainpoint of the substrate, preferably 400° C. or higher and lower than thestrain point of the substrate.

The first heat treatment at temperatures of 350° C. or higher allowsdehydration or dehydrogenation of the oxide semiconductor layer,resulting in a reduction in the hydrogen concentration in the layer. Thefirst heat treatment at temperatures of 450° C. or higher allows afurther reduction in the hydrogen concentration in the layer. The firstheat treatment at temperatures of 550° C. or higher allows a stillfurther reduction in the hydrogen concentration in the layer.

As the atmosphere in which the first heat treatment is performed, it ispreferable to use an inert gas that contains nitrogen or a rare gas(e.g., helium, neon, or argon) as its main component and that does notcontain water, hydrogen, or the like. For example, the purity of the gasintroduced to the heat treatment apparatus can be 6N (99.9999%) or more,preferably 7N (99.99999%) or more. In this manner, the oxidesemiconductor layer 431 is not exposed to the air during the first heattreatment so that the entry of water or hydrogen can be prevented.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for a heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by aheat treatment, such as nitrogen or a rare gas such as argon is used.

In this embodiment, a heat treatment is performed using a GRTA apparatusin a nitrogen atmosphere at 650° C. for 6 minutes as the first heattreatment.

The first heat treatment performed on the oxide semiconductor layer maybe performed on the oxide semiconductor film 430 which has not yet beenprocessed into the island-shaped oxide semiconductor layer. In thatcase, the second photolithography step is performed after the first heattreatment.

After that, a conductive layer is formed so as to cover the gateinsulating layer 402 and the oxide semiconductor layer 431 and is etchedby a third photolithography step, so that the source/drain electrodelayers 415 a and 415 b are formed (see FIG. 8C).

As a material for the conductive layer, a metal selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten; anitride containing any of these metal elements as a component; an alloycontaining any of these metals as a component; or the like can be used.A material selected from manganese, magnesium, zirconium, beryllium, andyttrium also may be used. Aluminum containing one or more of metalsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

The conductive layer may be formed using an oxide conductive film. Asthe oxide conductive film, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), a mixed oxide of indium oxide and tin oxide (In₂O₃—SnO₂,which is abbreviated to ITO in some cases), a mixed oxide of indiumoxide and zinc oxide (In₂O₃—ZnO), or any of these oxide conductivematerials which contains silicon or silicon oxide can be used.

In that case, as a material of the oxide conductive film, a materialwhose conductivity is high or whose resistivity is low as compared witha material used for the oxide semiconductor layer 431 is preferablyused. The conductivity of the oxide conductive film can be increased byan increase in the carrier concentration. The carrier concentration inthe oxide conductive film can be increased by increasing hydrogenconcentration or oxygen deficiency.

The source/drain electrode layers 415 a and 415 b can have asingle-layer structure or a stacked structure including two or morelayers.

In this embodiment, a first titanium layer having a thickness of 100 nm,an aluminum layer having a thickness of 400 nm, and a second titaniumlayer having a thickness of 100 nm are formed in this order over theoxide semiconductor layer 431. Then, the stacked-layer film includingthe first titanium layer, the aluminum layer, and the second titaniumlayer is etched so that the source/drain electrode layers 415 a and 415b are formed (see FIG. 8C).

The first heat treatment performed on the oxide semiconductor layer maybe performed after the source/drain electrode layers are formed. In thecase where the first heat treatment is performed after the source/drainelectrode layers are formed, a conductive layer having heat resistancefor this heat treatment is employed.

Note that materials and etching conditions are adjusted as appropriateso that the oxide semiconductor layer 431 is not removed by etching ofthe conductive layer.

Note that, in the third photolithography step, part of the oxidesemiconductor layer 431 is etched, whereby an oxide semiconductor layerhaving a groove (a depressed portion) is formed in some cases.

Next, a plasma treatment using a gas such as nitrous oxide (N₂O),nitrogen (N₂), or argon (Ar) is performed. By this plasma treatment,absorbed water and the like attached to an exposed surface of the oxidesemiconductor layer are removed. A plasma treatment may be performedusing a mixed gas of oxygen and argon.

After the plasma treatment, an oxide insulating layer 416 which servesas a protective insulating film and is in contact with part of the oxidesemiconductor layer is formed without exposure of the oxidesemiconductor layer to the air (see FIG. 8D).

The oxide insulating layer 416 can be formed by a method such as asputtering method, by which impurities such as hydrogen and water areprevented from entering the oxide insulating layer 416. The thickness ofthe oxide insulating layer 416 can be at least 1 nm or more. Whenhydrogen is contained in the oxide insulating layer 416, there is apossibility that entry of the hydrogen to the oxide semiconductor layer431 is caused, a back channel of the oxide semiconductor layer 431 ismade to have lower resistance (have n-type conductivity), and aparasitic channel is formed. Therefore, it is important that a formationmethod in which hydrogen is not used is employed in order to form theoxide insulating layer 416 containing as little hydrogen as possible.

The substrate temperature at the time of deposition of the oxideinsulating layer 416 may be higher than or equal to room temperature andlower than or equal to 300° C. The atmosphere in which the oxideinsulating layer 416 is formed can be a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas(typically argon) and oxygen.

In this embodiment, the substrate is heated at 200° C. before the oxideinsulating layer 416 is formed and a silicon oxide film as the oxideinsulating layer 416 is formed to a thickness of 300 nm so as to coverthe source/drain electrode layers 415 a and 415 b. The silicon oxidefilm is formed by a sputtering method using a silicon target and oxygenas a sputtering gas.

Next, a second heat treatment is performed in an inert gas atmosphere oran oxygen gas atmosphere (preferably at temperatures higher than orequal to 200° C. and lower than or equal to 400° C., for example attemperatures higher than or equal to 250° C. and lower than or equal to350° C.). For example, the second heat treatment is performed at 250° C.for 1 hour in a nitrogen atmosphere. Through the second heat treatment,part of the oxide semiconductor layer (a channel formation region) isheated while being in contact with the oxide insulating layer 416. Bythe second heat treatment, oxygen is supplied to the part of the oxidesemiconductor layer (the channel formation region). Thus, theconductivity type of a channel formation region 413 which overlaps withthe gate electrode layer 411 can be made to close to an i-type. Throughthe above-described steps, the transistor 410 is formed.

A protective insulating layer 403 may be formed over the oxideinsulating layer 416 (see FIG. 8D). For example, a silicon nitride filmcan be formed by an RF sputtering method. Since an RF sputtering methodhas high productivity, it is preferably used as a film formation methodof the protective insulating layer. It is preferable that the protectiveinsulating layer do not contain impurities such as moisture, hydrogenions, and OH⁻ and be formed using an inorganic insulating film whichprevents entry of these species from the outside.

Further, a heat treatment may be performed at temperatures higher thanor equal to 100° C. and lower than or equal to 200° C. for longer thanor equal to 1 hour and shorter than or equal to 30 hours in the air.This heat treatment may be performed at a fixed heating temperature.Alternatively, the following change in the heating temperature may berepeated plural times: the heating temperature is increased from roomtemperature to predetermined temperatures higher than or equal to 100°C. and lower than or equal to 200° C., and then decreased to roomtemperature. This heat treatment may be performed before formation ofthe oxide insulating film under reduced pressure. When the heattreatment is performed under reduced pressure, the heat treatment timecan be shortened. By the heat treatment, hydrogen can be taken in theoxide insulating layer 416 from the oxide semiconductor layer 431. Inother words, more hydrogen can be removed from the oxide semiconductorlayer.

A gate bias-temperature stress test (BT test) under conditions such as85° C., 2×10⁶ V/cm, and 12 hours does not show any change in theelectrical characteristics, which means that stable electricalcharacteristics are obtained.

The transistor using the oxide semiconductor which is described in thisembodiment has an electric characteristic of extremely low off currentwhen compared with a transistor using silicon or the like.

Therefore, by using the transistor described in this embodiment as thetransistor 207 illustrated in FIG. 1 or FIG. 3, the charge stored in thegate of the transistor 205 illustrated in FIG. 1 or FIG. 3 can be heldfor a long time. Accordingly, the charge stored in the gate of thetransistor 205 can be held almost constant during a period during whichreading of the charge stored in the gate of the transistor 205 isperformed a plurality of times. Thus, by using the oxide semiconductorlayer in the channel formation region of the transistor 207, asemiconductor device which includes a photosensor with a novel circuitstructure can be provided.

Further, by using the transistor described in this embodiment as thetransistor 201 illustrated in FIG. 3, an image signal holding period ofa pixel can be extended. Therefore, the size of a capacitor provided ina pixel can be reduced. Thus, the aperture ratio of the pixel can behigh, and an image signal can be input to the pixel at high speed, forexample. In addition, a rewriting interval of an image signal in stillimage display can be longer. For example, a writing interval of an imagesignal can be 10 seconds or longer, 30 seconds or longer, or 1 minute orlonger and shorter than 10 minutes. The longer the writing interval is,the more the power consumption can be reduced.

In addition, by using the transistor described in this embodiment as thetransistor 206 illustrated in FIG. 1 or FIG. 3, an unnecessary potentialcan be prevented from being output to the photosensor output signal line214 illustrated in FIG. 3 during a reading period for another pixel.

Further, the transistor described in this embodiment to which a backgate is added can be used as the transistor 205 illustrated in FIG. 1 orFIG. 3. Thus, an unnecessary potential can be prevented from beingoutput to the photosensor output signal line 214 during a reading periodfor another pixel.

An example of a cross-sectional view of the transistor provided with theback gate is illustrated in FIG. 9. A transistor 420 illustrated in FIG.9 has a structure of the transistor 410 illustrated in FIG. 8D to whicha back gate 441 is added. That is, the transistor 420 illustrated inFIG. 9 includes, over the substrate 400 having an insulating surface, agate electrode layer 411, the gate insulating layer 402, the oxidesemiconductor layer 431 having the channel formation region 413, thesource/drain electrode layers 415 a and 415 b, the oxide insulatinglayer 416, the back gate 441, and the protective insulating layer 403.

The back gate 441 can be formed using a material similar to the materialwhich can be used for the gate electrode layer 411. The oxide insulatinglayer 416 which is provided between the back gate 441 and the channelformation region can function as a gate insulating layer on the backgate 441 side. Therefore, the thickness of the oxide insulating layer416 can be approximately the same as that of the gate insulating layer402. Other parts of the structure can be similar to those of thetransistor 410 illustrated in FIG. 8D.

This embodiment can be implemented in combination with any of otherembodiments and an example as appropriate.

Example 1

In this example, evaluation of a transistor using an oxide semiconductorwhich is included in a semiconductor device of an embodiment of thedisclosed invention is described with reference to FIG. 10. In thisexample, measured values of the off current using a test element group(also referred to as a TEG) are described below.

The test element group was manufactured by connecting 200 transistorseach with a relationship between the length and width of the channelformation region of L/W=3 μm/50 μm in parallel. This test element groupcorresponds to a transistor with L/W=3 μm/10000 μm. The initialcharacteristics of the transistor manufactured as the test element groupare shown in FIG. 10. In the transistor, a highly purified oxidesemiconductor layer is used in the channel formation region. Note that alength (L_(ov)) of a region where a source electrode layer or a drainelectrode layer overlaps with the oxide semiconductor layer, in achannel length direction was 1.5 μm.

In order to measure the initial characteristics of the transistor, theV_(g)-I_(d) characteristics were evaluated by measuring the change insource-drain current (hereinafter, referred to as drain current orI_(d)) under the conditions where the substrate temperature was roomtemperature, the voltage between the source and the drain (hereinafter,referred to as drain voltage or V_(d)) was 1 V or 10 V, and the voltagebetween the source and the gate (hereinafter, referred to as a gatevoltage or V_(g)) was changed from −20 V to +20 V. Note that, in FIG.10, the V_(g)-I_(d) characteristics are shown in the range of V_(g) from−20 V to +5 V.

As illustrated in FIG. 10, the transistor with a channel width W of10000 μm has off current of 1×10⁻¹³ [A] or less at V_(d) of 1 V and 10V, which is less than or equal to the detection limit of a measurementdevice (a semiconductor parameter analyzer, Agilent 4156C manufacturedby Agilent Technologies Inc.). That is, it can be confirmed that offcurrent of the transistor per 1 μm of channel width is 10 aA/μm orlower. In addition, in the case where the channel length is 3 μm ormore, off current of the transistor is estimated to 10 aA/μm or lower.

Further, a transistor with a channel width W of 1000000 μm (1 m) wasmanufactured and subjected to the test. As a result, it was observedthat off current was 1×10⁻¹² [A] or less, which is near the detectionlimit. That is, it was revealed that off current of the transistor per 1μm of channel width is 1 aA/μm or lower.

The reason for the low off current of the transistor as low as 1×10⁻¹³[A] as shown in FIG. 10 is that hydrogen concentration in the oxidesemiconductor layer is sufficiently reduced in the above manufacturingprocess.

The carrier density of the oxide semiconductor layer which is measuredby a carrier measurement device is lower than 1×10¹²/cm³ or lower than1×10¹¹/cm³. That is, the carrier density of the oxide semiconductorlayer can be made as close to zero as possible.

Accordingly, the channel length L of the transistor can be greater thanor equal to 10 nm and less than or equal to 1000 nm Thus, operation rateof a circuit can be increased. Further, since the off current isextremely low, the power consumption can be reduced.

In addition, in circuit design, the oxide semiconductor layer can beregarded as an insulator when the transistor is in an off state.

A transistor using a highly purified oxide semiconductor (purified OS)as described above shows almost no dependence of off current ontemperature. It is considered that an oxide semiconductor does not showtemperature dependence when purified because the conductivity type ismade to extremely close to an intrinsic type and the Fermi level islocated in the middle of the forbidden band. This feature also resultsfrom the fact that the oxide semiconductor has a large band gap andincludes very few thermally excited carriers.

The above-described results show that off current of a transistor whosecarrier density is lower than 1×10¹²/cm³ or 1×10¹¹/cm³ is 1 aA/μm orlower in room temperature. In addition, by applying this transistor as atransistor included in a semiconductor device, a semiconductor deviceincluding a photosensor with a novel circuit structure can be provided.Further, power consumption of the semiconductor device can be reducedand display degradation (reduction in display quality) can besuppressed. Moreover, a semiconductor device can be provided in whichdisplay degradation (display change) due to an external factor such astemperature can be reduced.

This application is based on Japanese Patent Application serial no.2010-028762 filed with Japan Patent Office on Feb. 12, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: aphotodiode configured to receive a light; a first transistor comprisinga gate and a back gate; and a second transistor, wherein one of a sourceand a drain of the second transistor is directly connected to thephotodiode, wherein the other of the source and the drain of the secondtransistor is directly connected to the gate of the first transistor,wherein the photodiode is configured to generate an electric signal inaccordance with intensity of the light, wherein the first transistor isconfigured to convert a charge in the gate of the first transistor intoan output signal, wherein the first transistor is turned on when a firstpotential is input to the gate of the first transistor, and wherein thecharge is changed in accordance with the electric signal.
 2. Thesemiconductor device according to claim 1, wherein the first transistorfurther comprises an oxide semiconductor layer comprising a channelformation region, wherein the gate is over a substrate, wherein theoxide semiconductor layer is over the gate with a gate insulating layerinterposed therebetween, and wherein the back gate is over the oxidesemiconductor layer with an oxide insulating layer interposedtherebetween.
 3. The semiconductor device according to claim 1, whereina potential of the back gate is changed in accordance with a signalsupplied to a back gate signal line electrically connected to the backgate.
 4. The semiconductor device according to claim 1, wherein thefirst transistor is configured such that a potential of the back gate isvariable.
 5. The semiconductor device according to claim 4, wherein thesecond transistor comprises an oxide semiconductor layer comprising achannel formation region.
 6. The semiconductor device according to claim4, further comprising a third transistor, wherein one of a source and adrain of the third transistor is electrically connected to the gate ofthe first transistor, and wherein the other of the source and the drainof the third transistor is electrically connected to one of a source anda drain of the first transistor.
 7. The semiconductor device accordingto claim 6, further comprising a fourth transistor, wherein one of asource and a drain of the fourth transistor is electrically connected tothe other of the source and the drain of the first transistor.
 8. Thesemiconductor device according to claim 6, further comprising a fourthtransistor, wherein the other of the source and the drain of the thirdtransistor is electrically connected to the one of the source and thedrain of the first transistor via the fourth transistor, wherein one ofa source and a drain of the fourth transistor is electrically connectedto the other of the source and the drain of the third transistor, andwherein the other of the source and the drain of the fourth transistoris electrically connected to the one of the source and the drain of thefirst transistor.
 9. An electronic appliance comprising thesemiconductor device according to claim
 1. 10. A semiconductor devicecomprising: a photodiode configured to receive a light; a firsttransistor comprising a gate and a back gate; and a second transistor,wherein one of a source and a drain of the second transistor is directlyconnected to the photodiode, wherein the other of the source and thedrain of the second transistor is directly connected to the gate of thefirst transistor, wherein the first transistor is configured to converta charge in the gate of the first transistor into an output signal, andwherein the output signal is an electric signal in accordance withintensity of the light.
 11. The semiconductor device according to claim10, wherein the first transistor further comprises an oxidesemiconductor layer comprising a channel formation region, wherein thegate is over a substrate, wherein the oxide semiconductor layer is overthe gate with a gate insulating layer interposed therebetween, andwherein the back gate is over the oxide semiconductor layer with anoxide insulating layer interposed therebetween.
 12. The semiconductordevice according to claim 10, wherein a potential of the back gate ischanged in accordance with a signal supplied to a back gate signal lineelectrically connected to the back gate.
 13. The semiconductor deviceaccording to claim 10, wherein the first transistor is configured suchthat a potential of the back gate is variable.
 14. The semiconductordevice according to claim 13, wherein the second transistor comprises anoxide semiconductor layer comprising a channel formation region.
 15. Thesemiconductor device according to claim 13, further comprising a thirdtransistor, wherein one of a source and a drain of the third transistoris electrically connected to the gate of the first transistor, andwherein the other of the source and the drain of the third transistor iselectrically connected to one of a source and a drain of the firsttransistor.
 16. An electronic appliance comprising the semiconductordevice according to claim
 10. 17. A semiconductor device comprising: aphotodiode; a first transistor comprising a gate and a back gate; and asecond transistor, wherein one of a source and a drain of the secondtransistor is directly connected to the photodiode, wherein the other ofthe source and the drain of the second transistor is directly connectedto the gate of the first transistor, wherein the gate of the firsttransistor is electrically connected to the photodiode, and wherein thefirst transistor is configured to convert a charge in the gate of thefirst transistor into an output signal.
 18. The semiconductor deviceaccording to claim 17, wherein the first transistor further comprises anoxide semiconductor layer comprising a channel formation region, whereinthe gate is over a substrate, wherein the oxide semiconductor layer isover the gate with a gate insulating layer interposed therebetween, andwherein the back gate is over the oxide semiconductor layer with anoxide insulating layer interposed therebetween.
 19. The semiconductordevice according to claim 17, wherein a potential of the back gate ischanged in accordance with a signal supplied to a back gate signal lineelectrically connected to the back gate.
 20. The semiconductor deviceaccording to claim 17, further comprising a second transistor, whereinthe first transistor is configured such that a potential of the backgate is variable.
 21. The semiconductor device according to claim 20,wherein the second transistor comprises an oxide semiconductor layercomprising a channel formation region.
 22. The semiconductor deviceaccording to claim 20, further comprising a third transistor, whereinone of a source and a drain of the third transistor is electricallyconnected to the gate of the first transistor, and wherein the other ofthe source and the drain of the third transistor is electricallyconnected to one of a source and a drain of the first transistor.
 23. Anelectronic appliance comprising the semiconductor device according toclaim 17.